Besides punishment, the other key mechanism in sustaining cooperation towards collective action is reputation [1]}. Adopting a loose definition, reputation emerges when an individual's actions can be directly or indirectly observed by his peers and used to condition their own behaviour when playing with him. In both well-mixed [2]}, [3]} and structured populations [4]}, simple reputational systems promote and stabilise cooperation. Reputational systems at different levels of complexity have been studied, ranging from the first-order image scoring norm [5]}, to the second-order standing criterion [6]}, [7]} and to the more complex third-order leading eight social norms [8]}, [9]}. While simpler reputational systems have been observed in animals [10]}, more elaborate systems are more likely to be the domain of human interactions, due to the complex relationship between actions, reputations and social structures.
How reputation evolves when a given behaviour is observed depends on the social norm characterising a population, in other terms their “notion of goodness”, i.e. from the moral value attributed to a given type of action.
